Capabilities of the MR Instrument

MR is a dedicated instrument for the investigation of magnetic and nonmagnetic nanosystems. It uses in combination three principle grazing-incidence methodologies: reflectometry, off-specular scattering, and grazing-incidence small-angle neutron scattering (GISANS).

  • Grazing-Incidence Scattering is a powerful method for studying magnetism in nanosystems. Scientific and technological applications of artificially created magnetic structures with reduced dimensionalities are of ever-growing interest. Systems that can be studied include contemporary magnetic materials for recording devices, thin films, and multilayers with the interlayer magnetic coupling, bias effects, distribution of magnetic moments, magnetic dead layers, impurity diffusion, and interlayer roughness (chemical and magnetic).
  • Specular Polarized and Unpolarized Neutron Reflectometry (PNR and NR) probe the depth-dependent composition in thin films, multilayers, and nanostructures. PNR is sensitive to the magnetic, as well as the nuclear (or chemical), composition of samples.
  • Specular reflection probes the depth profile of the mean scattering length density (SLD) averaged over the lateral projection of the coherence length. Specular reflection is measured in slit-like collimation.
  • Off-specular scattering, accompanying PNR, is sensitive to magnetic and structural correlations in the plane of thin-film samples, making it a powerful technique for investigating patterned magnetic structures, magnetic domains, and lateral correlations. The off-specular scattering probes the lateral fluctuations within the reflection plane in the micro- to sub-millimeter range.
  • Off-specular scattering measures the lateral fluctuations of the SLD around its mean value. At shallow incidence, off-specular scattering, measured in slit-like collimation, probes large lateral scales (micro- to sub-millimeter range).
  • Grazing-Incidence Small-Angle Neutron Scattering (GISANS). GISANS probes the lateral fluctuations perpendicular to the reflection plane in the nano- to micro-millimeter range. GISANS is performed in a point-like or pencil collimation.

The versatility of MR is illustrated by the following examples of materials studied that include magnetic nanostructures, thin films and multilayers, biological membranes, and polymer/nanocomposite films. The number of days to collect the neutron data for different experiments varies depending on the lateral size of the sample, the configuration of the instrument, the collimation determined by the scientific case, and the resolution and Q range needed.

Magnetic thin films, multilayers, nanostructures

Interfacial magnetism in complex oxide heterostructures determined with PNR

Oxygen vacancy distributions and dynamics directly control the operation of solid-oxide fuel cells and are intrinsically coupled with magnetic, electronic, and transport properties of oxides. To understand the atomistic mechanisms involved during operation of the cell, it is highly desirable to know the distribution of vacancies on the unit cell scale. Here, we developed an approach for direct mapping of oxygen vacancy concentrations based on local lattice parameter measurements. PNR reveals strong differences in magnetic properties of lanthanum strontium cobaltite model systems induced by different symmetries of the substrates. (Sample size: 10 x 10 mm; polarized neutrons; 3 to 4 hours per neutron spin-state; 4 days in total)

Kim Y.-M., He J., Biegalski M. D., Ambaye H., Lauter V., Christen H. M., Pantelides S. T., Pennycook S. J., Kalinin S. V., Borisevich A. Y., "Probing oxygen vacancy concentration and homogeneity in solid-oxide fuel-cell cathode materials on the subunit-cell level", Nature Materials, 11, 888-894, (2012).

O'Brien L., Erickson M.J., Spivak D., Ambaye H., Goyette R.J., Lauter V., Crowell P.A., Leighton C., "Kondo physics in non-local metallic spin transport devices", Nature Communications, 5, 49210, (2014).

Biegalski M.D., Takamura Y., Mehta A., Gai Z., Kalinin S.V., Ambaye H., Lauter V., Fong D., Pantelides S.T., Kim Y.M., He J., Borisevich A., Siemons W., Christen H.M., "Interrelation between Structure - Magnetic Properties in La0.5Sr0.5CoO3", Advanced Materials Interfaces, 1, 1400203, (2014).

Spurgeon S. R., Sloppy J. D., Kepaptsoglou D. M., Balachandran P. V., Nejati S., Karthik J., Damodaran A. R., Johnson C. L., Ambaye H., Goyette R., Lauter V., Ramasse Q. M., Idrobo J. C., Lau K. K.S., Jr. S. E.Lofland, Rondinelli J. M., Martin L. W., Taheri M. L., "Thickness-dependent crossover from charge- to strain-mediated magnetoelectric coupling in ferromagnetic / piezoelectric oxide heterostructures", ACS Nano, 8, 1, 894-903, (2014).

Chen S.W., Lu X., Blackburn E., Lauter V., Ambaye H., Chan K.T., Fullerton E.E., Berkowitz A.E., Sinha S.K., "Nonswitchable magnetic moments in polycrystalline and (111)-epitaxial permalloy/CoO exchange-biased bilayers", Physical Review B: Condensed Matter and Materials Physics, 89, 094419, (2014).

Tronin A.Y., Nordgren C.E., Strzalka J.W., Kuzmenko I., Worcester D.L., Lauter V., Freites J.A., Tobias D.J., Blasie J.K., "Direct evidence of conformational changes associated with voltage gating in a voltage sensor protein by time-resolved X-ray/neutron interferometry", Langmuir, 30, 4784-4796, (2014).

Ke X., Belenky L. J., Lauter V., Ambaye H., Bark C. W., Eom C. B., Rzchowski M. S., "Spin structure in an interfacially coupled epitaxial ferromagnetic oxide heterostructure ", Physical Review Letters, 110, 237201, (2013).

Schumacher D., Steffen A., Voigt J., Schubert J., Bruckel T., Ambaye H., Lauter V., "Inducing exchange bias in La0.67Sr0.33MnO3-delta/SrTiO3 thin films by strain and oxygen deficiency", Physical Review B: Condensed Matter and Materials Physics, 88, 144427, (2013).

The magnetic tunnel junction (MTJ) with a perpendicular magnetic anisotropy (PMA) for a small critical current density for current-induced magnetization switching―a pathway for current-induced spin transfer torque devices

The PMA in the CoFeB sandwiched by MgO and Ta layers has been investigated by using anomalous Hall effect and PNR. It is found that the interfacial anisotropies at both MgO/CoFeB and CoFeB/Ta interfaces contribute to the observed PMA. A large PMA in the MgO/CoFeB/Ta trilayers is also related to the low magnetization of CoFeB deposited on MgO, which is confirmed by PNR measurements. (Sample size: 12 x 12 mm, polarized neutrons, 3 to 4 hours per neutron spin-state, 4 to 5 days in total)

T. Zhu, Y. Yang, R. C. Yu, H. Ambaye, and Valeria Lauter, “Large anomalous Hall effects in a ferromagnetic layer sandwiched by MgO and Ta layers,” Applied Physics. Letters 100, 202406 (2012).

Giant saturation magnetization Fe16N2 films: the combination of the large magnetocrystalline anisotropy and its giant saturation magnetization gives us a new direction for the synthesis of rare-earth-element free magnets in the future.

We have demonstrated a general framework for realizing and modulating perpendicular magnetic anisotropy in a rare-earth-element and heavy-metal-free material system. Using GaAs(001)/Fe(001) template, we have developed a synthesis scheme to produce epitaxial body center tetragonal Fe-N with (001) texture. By varying the N doping concentration, the crystal tetragonality (c/a) can be tuned in a relatively wide range. It is found that the Fe-N layer developes a strong perpendicular magnetic crystalline anisotropy (MCA) as it approaches the iron nitride interstitial solubility limit. Further annealing significantly improves the MCA due to the formation of chemically ordered Fe16N2. In addition to realizing an MCA up to 107 erg/cm3, the spin polarization ratio (P ∼ 0.52), as probed directly by a point-contact Andreev reflection (PCAR) method, even shows a moderate increase in comparison with normal metal Fe (P ∼0.45). (Sample size: 10 x 10 mm, polarized neutrons, 3 to 4 hours per neutron spin-state, 3 to 4 days in total)

Nian Ji, M. S. Osofsky, Valeria Lauter, Lawrence F. Allard, Xuan Li, Kevin L. Jensen, H. Ambaye, Edgar Lara-Curzio and Jian-Ping Wang, “Perpendicular magnetic anisotropy and high spin polarization ratio in epitaxial Fe-N thin films,” Physical Review B 84, (2011) 245310.

Intrinsic magnetic properties of hard-soft heterostructures.

Interestingly, for Fe/Sm-Co spring magnets, the magnetic properties of interfaces play critical roles in achieving the synergistic function of magnetic heterostructures. In particular, it has been reported that compositionally intermixed interfaces may enhance the energy storage density of exchange-spring magnets consisting of high-magnetization but soft Fe phase and high-anisotropy Sm-Co phase, which can be potentially used as ultra-strong permanent magnets. The nanoscale variations of the micromagnetic properties at the interface are believed to be the underlying mechanism but could not yet be probed in a quantitative manner. Here we show that it is possible to determine quantitative, spatially resolved profiles of micromagnetic properties from complementary studies of polarized neutron reflectometry (PNR) and micromagnetic simulations.

Y. H. Liu, J. S. Jiang, H. Ambaye, S. G. E. te Velthuis, “Magnetization profiles in Fe/SmCo Spring Magnets with Graded Interfaces,” Physical Review B 83, (2011) 174418. (4 Days)

Organic Spin-Valve Multilayers

The electronic, magnetic, and structural properties of Ni80Fe20 and Co electrodes at LiF and aluminum tris(8-hydroxyquinoline), or Alq3, interfaces were investigated with photoemission spectroscopy and polarized neutron reflectivity measurements. When LiF was deposited onto Ni80Fe20 films and Co was deposited onto thin LiF layers, the work function of both metals decreased. Polarized neutron reflectivity measurements were used to probe the buried interfaces of multilayers resembling a spin-valve structure. The results indicate that LiF is an effective barrier layer to block diffusion of Co into the Alq3 film.

Greg Szulczewski, Jonathan Brauer, Edward Ellingsworth, Justin Kreil, H. Ambaye, and V. Lauter, “The role of LiF tunnel barriers on the performance of organic spin-valves” Journal of Applied Physics 109, 07C509 (2011). (2 to 3 Days)

Shape memory alloy

The first results from in situ polarized neutron diffraction experiments on a time-of-flight instrument at a spallation source. Magnetic-field-induced rotation of electron spins in a Ni-Mn-Ga single crystal was measured. The difference in intensities measured with polarized neutron spin-up and spin-down is proportional to the field-induced magnetization of the crystal. The polarized neutron measurements indicate that the magnetic form factor for the 3d electrons of Mn in Ni-Mn-Ga is lower than the value reported earlier for an ideal spherical symmetry of electronic distribution.

A. Pramanick, V. Lauter, X.-L. Wang, K. An, H. Ambaye, R. Goyette, J. Yi, Z. Gai, and A. D. Stoica, “Polarized neutron diffraction at a spallation source for in-situ magnetization studies,” Journal of Applied Crystallography, submitted. (1 to 2 Days including the modification of the configuration for diffraction experiment)

Materials Science

Polyelectrolyte/Nanoparticle Multilayers

Here, we demonstrate the exponential LbL (e-LbL) growth of poly(ethyleneimine)/SiO2 nanoparticles (PEI/SiO2) bicomponent thin films that consist mostly of SiO2 nanoparticles (over 90 wt % obtained by thermogravimetric analysis). We show that the LbL growth of the PEI/SiO2 system significantly depends on the pH of the PEI and the SiO2 solutions. The e-LbL growth will only occur when the film is deposited with PEI at a high pH and SiO2 at a low pH. The exponential growth was characterized using a quartz crystal microbalance, atomic force microscopy and scanning electron microscopy imaging, and neutron reflectometry.

Chunqing Peng; Yonathan S. Thio; Rosario A. Gerhardt, Haile Ambaye, and Valeria Lauter, “pH-Promoted Exponential Layer-by-Layer Assembly of Bicomponent Polyelectrolyte/Nanoparticle Multilayers,” Chemistry of Materials 23, 4548 (2011). (4 Days)

Layered Organic compounds on Mineral Surfaces

Organic carbon (OC) stabilization in soils plays a significant role in the global C cycle; therefore, the understanding of the structure and function of the OC-soil mineral interface is of high importance. To study the interface, films of simple OC compounds and natural organic matter (NOM) were deposited onto a soil mineral analogue (Al2O3) using spin coating and were exposed to humidity. The thickness, density, and structure of the films were studied using a depth-sensitive technique of neutron reflectivity.

Melanie Mayes, Sindhu Jagadamma, Haile Ambaye, Loukas Petridis, and Valeria Lauter, “Neutron Reflectometry Reveals the Internal Structure of Natural Organic Matter Deposited onto an Aluminum Oxide,” Geoderma, submitted. (3 Days)